This invention relates to durable fixing-unit arranged to obtain high quality toner images by detecting surface temperature of a heating member through a contact-type temperature detecting sensor or image forming apparatus incorporating this fixing-unit.
In the prior art, it is generally known and used in an image forming apparatus that after transcribing toner images on a photosensitive material onto a recording medium, the toner images are fixed on the recording medium by heating and being melted with the fixing-unit employing a heating roller and a compression roller. In general, fixing temperature is controlled by a contact-type or non-contact type temperature sensor, which detects temperature of the heating roller, and set in the vicinity of the heating roller within the fixing-unit.
Contact-type temperature sensors are widely used in image forming apparatus that requires high-speed processing and short warm-up time, from the viewpoint of securing durability and highly accurate temperature detection, compared to non-contact type temperature sensors. In this case, it is common that a temperature sensor having a heat sensitive element is arranged to contact the surface of a cylindrical heating roller along the radial direction of the heating roller to enable the temperature sensor to respond quickly.
Tokkai 2002-304084 (FIG. 4)
Tokkai 2001-5333 (FIG. 2)
However, contact condition between the surface of the heating roller and the temperature sensor worsens since residual toner, which adheres to the heating roller and paper powder of the recording medium adheres to the surface of the temperature sensor. It often happens that the detected temperature by the temperature sensor indicates a lower temperature than the actual temperature even though the temperature reaches the pre-determined temperature at which the toner can be fixed onto the recording medium.
Based on the above fact, there are cases in which toner copied onto a recording medium cannot be fixed on the recording medium due to the higher temperature of the heating roller than the temperature that is required and some of the toner adheres to the heating roller even when pressure is applied by the pressing roller, which is called, high temperature offset. Further, there are cases in which jamming problems occur due curled recording media.
To solve the above problems, there has been one proposal that cleaning unit, which automatically wipes contacting surfaces of the heat sensitive element of the temperature sensor. But there is a drawback that the equipment itself becomes more complex and costly. On the other hand, the adhesive force of the toner or powdered paper debris to the temperature sensor has been increasing and it has become more difficult to remove the adhesive toner with solvent during maintenance.
For example, heat resistant film 17, such as polyimides, is used to cover the contacting surface of the temperature sensor in order to protect the sensor portion. However, leaving the adhesive toner on the film at high temperature for a long time results in rigid portion on the film due to a chemical reaction between toner and the film. Also it has become more difficult to wipe the toner and paper powder off the temperature sensor since the toner clings to scratches caused by the heat resistant film 17 rubbing against the heating roller.